I was flying UAL coach class out of LHR
last spring, so I can probably help. When you go through the small passage to all gates (not the passage with the people-movers (conveyor belt things)), turn left and walk allllllll the way down to the end of the terminal, and there will be an area with big floor to ceiling windows, good for spotting. I took these shots from this location.
the best part was that there was nobody at that end of the terminal. literally nobody.
before I got to the end of the terminal, I stopped midway (before I knew I could go further) and took some photos, as well.
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Photo © Msh744
here's a photo of the area at the end of the terminal.
hope I answered your question.